多线程数据保护-读写锁

发表于2021-07-06更新于2021-07-06

简述

读写锁是一种共享锁，在同一时间可以有多个线程获取读锁，但是只能有一个获取写锁。即读锁之间是共享的，但是写锁之间是互斥的，并且在写的时候不能进行读操作。

相关函数

作用 函数
初始化读写锁 pthread_rwlock_init
读取读写锁中的锁 pthread_rwlock_rdlock
读取非阻塞读写锁中的锁 pthread_rwlock_tryrdlock
写入读写锁中的锁 pthread_rwlock_wrlock
写入非阻塞读写锁中的锁 pthread_rwlock_trywrlock
解除锁定读写锁 pthread_rwlock_unlock
销毁读写锁 pthread_rwlock_destroy

注意点

如果一个进程中读锁过多，但是写锁很少，有时会导致写锁的饥饿情况，所以可以使用如下的函数来设置锁的优先级：

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int pthread_rwlockattr_setkind_np(pthread_rwlockattr_t *attr,
                              int pref);
int pthread_rwlockattr_getkind_np(
                              const pthread_rwlockattr_t *restrict attr,
                              int *restrict pref);

参考：

pthread_rwlockattr_setkind_np(3) - Linux manual page

练习

要求

  • 使用读写锁来保护数据
  • 在主进程里面调用两次读锁
  • 在一个子线程里面调用一次读锁
  • 在另一个子线程里面调用写锁
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#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

pthread_rwlock_t rwlock;

void checkRes(char * funName, int ret)
{
	if (ret)
		printf("Fun %s error %d\n",
			funName, ret);
}

void *rdlockThread(void *arg)
{
	int ret = 0;

	printf("Entered thread, getting read lock\n");
	ret = pthread_rwlock_rdlock(&rwlock);
	checkRes("pthread_rwlock_rdlock", ret);
	printf("got the rwlock read lock\n");

	sleep(5);

	printf("unlock the read lock\n");
	ret = pthread_rwlock_unlock(&rwlock);
	checkRes("pthread_rwlock_unlock", ret);
	printf("thread read lock unlocked\n");

	return NULL;
}

void *wrlockThread(void *arg)
{
	int ret = 0;
	printf("Entered thread, getting write lock\n");
	ret = pthread_rwlock_wrlock(&rwlock);
	checkRes("pthread_rwlock_wrlock", ret);

	printf("Got the rwlock write lock, now unlock\n");
	ret = pthread_rwlock_unlock(&rwlock);
	checkRes("pthread_rwlock_unlock", ret);
	printf("thread write lock unlock\n");

	return NULL;
}

int main(int argc, char **argv)
{
	int ret = 0;
	pthread_t pid_read, pid_write;
	printf("Enter Testcase - %s\n", argv[0]);

	printf("Main, init the rwlock\n");
	ret = pthread_rwlock_init(&rwlock, NULL);
	checkRes("pthread_rwlock_init\n", ret);

	printf("Main grab a read lock\n");
	ret = pthread_rwlock_rdlock(&rwlock);
	checkRes("pthread_rwlock_rdlock", ret);

	printf("Main grab the same read lock again\n");
	ret = pthread_rwlock_rdlock(&rwlock);
	checkRes("pthread_rwlock_rdlock", ret);

	printf("Main create the read lock thread\n");
	ret = pthread_create(&pid_read, NULL, rdlockThread, NULL);
	checkRes("pthread_create", ret);

	printf("Mian unlock first read lock\n");
	ret = pthread_rwlock_unlock(&rwlock);
	checkRes("pthread_rwlock_unlock", ret);

	printf("Mian create the write lock thread\n");
	ret = pthread_create(&pid_write, NULL, wrlockThread, NULL);
	checkRes("pthread_create", ret);

	sleep(5);

	printf("Main unlock the second read lock\n");
	ret = pthread_rwlock_unlock(&rwlock);
	checkRes("pthread_rwlock_unlock", ret);

	printf("Main wait for the threads\n");
	ret = pthread_join(pid_read, NULL);
	ret |= pthread_join(pid_write, NULL);
	checkRes("pthread_join", ret);

	ret = pthread_rwlock_destroy(&rwlock);
	checkRes("pthread_rwlock_destory", ret);

	printf("end\n");

	return 0;

}